Wood plastic composites (WPCs) represent a class of materials that are of great interest especially for construction and decorative application. Final products are for example decking boards for terraces or window and door frames. Wood plastic composites can be processed either by extrusion or injection molding. The majority of WPCs is prepared by extrusion of wood flour with thermoplastic resins such as polypropylene or polyethylene and optionally further additives.
One problem of wood plastic composites is their relatively high flammability as compared to normal untreated wood. The combination of wood flour with highly flammable polymers such as polyethylene and polypropylene worsens the fire performance and facilitates the ignitability of the compounded products. The application of fire retardant coatings to WPCs often fails because of adhesion problems between the coating and the polymeric material.
WPCs may contain additional ingredients that help maintaining the desired properties such as color, smoothness and processing characteristics. Therefore, it is also possible to use flame retardant additives when compounding the WPC material. A flame retardant additive is a chemical compound that helps reducing the ignitability and flame spread of a material. There numerous different flame retardant additives on the market based on different chemistries such as halogenated compounds, metal hydroxides, organic or inorganic phosphorus containing compounds and nitrogen-based compounds. As the WPC material is processed by extrusion the flame retardant additive must meet certain processing requirements such as thermal stability, inertness and compatibility with the polymer matrix.
A conference paper by Robert H. White, Nicole M. Stark and Nadir Ayrilmis, entitled “2011 Recent Advances in Flame Retardancy of Polymeric Materials”, BCC Research, Wellesley, Mass., May 23-25, 2011, includes a comparison of WPC materials comprising different flame retardant additives such as ammonium polyphosphate, melamine phosphate, zinc borate, magnesium hydroxide and decabromodiphenyl oxide/antimony trioxide. It was found that fire retardants are effective in WPCs, but that the addition of fire retardant chemicals can also have a detrimental effect on mechanical and moisture resistance properties of WPCs.
Accordingly, there is still a big demand for wood plastic composites containing efficient flame retardants.
Wooden materials and polymers based on polyolefins are often coated or blended with an intumescent flame retardant formulation to meet fire safety standards. An intumescent coating is usually composed of an acid source such as a phosphoric acid derivative, a blowing agent such as a nitrogen-containing compound, and a charring agent such as an organic compound having an increased number of hydroxyl groups. In case of a fire or contact to the flame these compounds react with each other to form a voluminous bulky char that protects the substrate from heat and oxygen supply and thus prevents a flame spread.
Since wood flour is based on cellulose which is a polyhydroxy material capable of forming a char, intumescent flame retardants should also be suitable for use with WPCs. However, intumescent flame retardants are quite reactive formulations so that they can cause problems during processing of WPCs at elevated temperatures due to degradation of the polymer matrix.